1. Cross References to Related Application
This application relates to and claims priority from Japanese Patent Application No. 2003-397380 filed on Nov. 27, 2003, the entire disclosure of which is incorporated herein by reference.
2. Field of the Invention
The present invention relates to a storage system, a storage control device, and a data relay method using the storage control device.
3. Description of the Related Art
For example, in a database system handling large scale data, such as information management systems of data centers or enterprises, the data are managed by using a storage system configured separately from a host computer. This storage system is constituted, for example, by disk array devices. A disk array device is configured by arranging a multiplicity of storage devices in the form of an array and is constructed based, for example, on RAID (Redundant Array of Independent Inexpensive Disks). A storage device group is constituted, for example, by hard disk devices or semiconductor memory devices and is connected by a SAN (Storage Area Network) or the like. At least one logical volume (logical unit) is formed on a physical storage area provided by the storage device group, and this logical volume is provided to the host computer. The host computer can conduct data writing/reading to/from the logical volume.
In the storage system, data loss is prevented and continuous operation is ensured by taking a variety of protection measures. One of them is the utilization of a RAID configuration. For example, the possibility of data loss can be reduced when the disk array device employs, for example, a redundant storage structure known as RAID 1 to RAID 6. Furthermore, duplication of physical configuration is also conducted in the disk array system. For example, in the disk array system, multiplexing is attained by providing a plurality of main components such as high-order interface circuits for conducting data communication with a host computer and low-order interface circuits for conducting data communication with disk drives. Furthermore, there are also provided a plurality of communication paths connecting the above-mentioned respective main components and a plurality of power sources for supplying electric power to the main components.
Furthermore, in the disk array devices, for example, the logical volumes of the RAID structure can be duplicated and the respective identical data can be stored in a pair of logical volumes, primary volume and auxiliary volume.
In recent years, remote systems have been sometimes provided in locations physically separated by a large distance from a local system, as a provision against unexpected situations such as natural disasters, this measure known as a disaster recovery. A copy of the primary volume used in the local system is stored in the remote system. Data can be transmitted from the local system to the remote system without using a host computer, and an auxiliary volume having the contents identical to that of the primary volume of the local system can be created in the remote system.
Data copying is conducted in two stages to match the contents of the primary volume and auxiliary volume. One stage is initial copying. In initial copying, the storage contents of the primary volume is copied in its entirety to the auxiliary volume. The other stage is differential copying. In differential copying, only data updated in the primary volume of the local system after the completion of initial copying is transferred to the auxiliary volume of the remote system. When the local system stops functioning due to a natural disaster or intentional attack, the operations of the local system are continued by the remote system till the local system is restored. The technology for copying the storage contents of a local disk device to an external disk device located at a distance from the local disk device is known as mirroring or remote copying (Japanese Patent Application Laid-open No. H11-338646).
Remote copying can be generally classified into synchronous and asynchronous remote copying. In the case of synchronous remote copying, after the data received from the host computer has been stored in a cache memory, this data is transferred to the remote system via a communication network (SAN, IP network, or the like). If the remote system receives the data and stores it in a cache memory, a response signal indicating data reception is transmitted to the local system. If the local system receives a response signal from the remote system, it posts to the host computer a write completion report notifying that the data writing was normally conducted. Thus, in the synchronous remote copying, the write request from the host computer and data transfer to the remote system are executed synchronously. Therefore, because a delay corresponding to a time period waiting for a response from the remote system occurs, such copying is suitable when the distance between the local system and remote system is comparatively short. Conversely, when the local system and remote system are at a large distance from each other, the synchronous remote copying is generally unsuitable because of the problems associated with response delay and propagation delay.
On the other hand, in the case of asynchronous remote copying, if the local system receives a write request from the host computer, it stores the received data in a cache memory and immediately sends a write completion report to the host computer. After sending the write completion report to the host computer, the local system transfers the data to the remote system. Thus, reporting the write completion to the host computer and transferring the data to the remote system are conducted asynchronously. Therefore, in the case of the asynchronous remote copying, the write completion report can be rapidly transmitted to the host computer, regardless of the distance between the local system and remote system. Therefore, the asynchronous remote copying is applicable to the cases when the local system and remote system are at a large distance from each other. However, in the case of an asynchronous remote copying, data transfer to the remote system has not yet been conducted when the write completion report is sent to the host computer. Therefore, though the write completion report has been sent, it cannot be guaranteed that the storage contents of the primary volume and the storage contents of the auxiliary volume are matched.
Synchronization of the storage contents of the local system and remote system is a measure for coping with the occurrence of natural disasters and increases the reliability of the storage system. For example, considering the occurrence of large-scale natural disasters affecting a wide area, such as large earthquakes, it is preferred that the local system and remote system be as far from each other as possible. This is because the shorter is the distance between the local system and the remote system, the higher is the probability that it will be impossible to cope with wide-area failure.
On the other hand, because of physical limitations of the communication path between the local system and remote system, the distance between the local system and remote system is naturally also limited. For example, when fiber channel transfer is conducted by using an optical fiber cable, the distance at which communication is possible is about several tens of kilometers, the specific distance depending on the aperture and mode of fiber cable and the like. Therefore, when data is transmitted over a large distance by using fiber channel transfer by an optical fiber cable, a relay system is disposed between the local system and remote system. Introducing a relay system makes it possible to increase, for example to double, the distance between the local system and remote system.
Furthermore, when the copy of data is saved only in one remote system, if by any chance the data copy of the remote system is lost due to some failure after the local system has stopped functioning, a long time will be required to restore the storage system. Therefore, it is preferred that the measures taken are not limited to disaster recovery and that the copies of the primary volume are saved in multiple locations.
If a relay system is disposed between the local system and the remote system, then the data of the local system can be saved in both the relay system and the remote system and the reliability is improved. Furthermore, because the distance between the local system and remote system is increased, the resistance to wide-range obstacles such as large earthquakes is also increased.
Further, in a storage system, as the system expands, old storage control devices are present together with new storage control devices. Accordingly, it is desirable that the new storage control devices and old storage control devices be employed in cooperation with each other.
The present invention was created to resolve the aforesaid problems and it is an object of the present invention to provide a storage system, a storage control device, and a data relay method using the storage control device that are comparatively inexpensive and can save a copy of a volume in a plurality of locations. It is another object of the present invention to provide a storage system, a storage control device, and a data relay method using the storage control device that can establish cooperation between a plurality of different storage control devices and realize a plurality of remote copies. Other objects of the present invention will be evident from the description of the preferred embodiments thereof provided hereinbelow.